Hydraulic fracture composition and method

ABSTRACT

A method for improving the performance of fracturing processes in oil production fields may rely on polymer coated particles carried in the fracturing fluid. The particles may include heavy substrates, such as sand, ceramic sand, or the like coated with polymers selected to absorb water, increasing the area and volume to travel more readily with the flow of fluid without settling out, or allowing the substrate to settle out. Ultimately, the substrate may become lodged in the fissures formed by the pressure or hydraulic fracturing, resulting in propping open of the fissures for improved productivity.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/136,352, filed Apr. 22, 2016, which is acontinuation-in-part of U.S. patent application Ser. No. 15/011,111,filed Jan. 29, 2016, which is a continuation-in-part of U.S. patentapplication Ser. No. 14/171,920, filed Feb. 4, 2014, which is acontinuation of U.S. patent application Ser. No. 13/418,227, filed Mar.12, 2012, now U.S. Pat. No. 9,057,014 issued Jun. 16, 2015; which is acontinuation-in-part of U.S. patent application Ser. No. 13/299,288,filed Nov. 17, 2011, now U.S. Pat. No. 8,661,729 issued Mar. 4, 2014;which is a continuation-in-part of U.S. patent application Ser. No.12/789,177, filed May 27, 2010, now U.S. Pat. No. 8,341,881 issued Jan.1, 2013; which is a continuation of U.S. patent application Ser. No.12/324,608, filed on Nov. 26, 2008 now U.S. Pat. No. 7,726,070, issuedJun. 1, 2010; which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/012,912, filed Dec. 11, 2007; all of which arehereby incorporated by reference in their entirety.

BACKGROUND

1. The Field of the Invention

This invention relates to oil field and oil well development, and, moreparticularly, to novel systems and methods for fracturing and proppingfissures in oil-bearing formations to increase productivity.

2. The Background Art

Oil well development has over one hundred years of extensive engineeringand chemical improvements. Various methods for stimulating production ofwell bores associated with an oil reservoir have been developed. Forexample, United States Patent Application Publication US 2009/0065253 A1by Suarez-Rivera et al. and entitled “Method and System for IncreasingProduction of a Reservoir” is incorporated herein by reference in itsentirety and provides a description of fracturing technology in order toincrease permeability of reservoirs. Moreover, various techniques existto further improve the fracture channels, such as by acid etching asdescribed in U.S. Pat. No. 3,943,060, issued Mar. 9, 1976 to Martin etal., which is likewise incorporated herein by reference in its entirety.

In general, different types of processes require various treatments. Ingeneral, well production can be improved by fracturing formations.Fracturing is typically done by pumping a formation full of a fluid,containing a large fraction of water, and pressurizing that fluid inorder to apply large surface forces to parts of the formation. Theselarge surface forces cause stresses, and by virtue of the massive areasinvolved, can produce extremely high forces and stresses in the rockformations. Accordingly, the rock formations tend to shatter, increasingporosity an providing space for the production oil to pass through theformation toward the bore hole for extraction. However, as the foregoingreferences describe, the chemistry is not simple, the energy and timerequired for incorporation of various materials into mixtures is time,money, energy, and other resource intensive.

It would be an advance in the art if such properties as viscosity,absorption, mixing, propping, and so forth could be improved by animproved composition and method for introduction.

Brief Summary of the Invention

In view of the foregoing, in accordance with the invention as embodiedand broadly described herein, a method, apparatus, and composition aredisclosed in certain embodiments in accordance with the presentinvention, as including a substrate that may be formed of sand, rockproduct, ceramic sand, gravel, or other hard and structurally strongmaterials, provided with a binder to temporarily or permanently secure ahydrating polymer in proximity to the substrated. When used herein anyreference to sand or proppant refers to any or all of these used inaccordance with the invention. In certain embodiments of a method inaccordance with the invention, a composition as described may be mixeddirectly into drilling fluids, such as a fracturing fluid made up ofwater and other additives.

By virtue of the increased surface area and weight provided to thepolymeric powders affixed to the substrate, the surface area, andconsequently the frictional drag, is greatly increased, sweeping thematerial of the invention into a flow of fluid. This greatly decreasesthe time required to absorb polymers into the fluid.

In fact, rather than having to wait to have the polymers thoroughlymixed, or absorb a full capacity of water, and thereby flow properlywith the drilling fluid or fracturing fluid, a composition in accordancewith the invention will sweep along with the fluid immediately, with theweight of the substrate submerging the polymer. Meanwhile, the crosssectional area presented results in hydrodynamic drag sweeps thecomposition along with the flow.

Meanwhile, over time, the polymeric powder adhered to the substrate willabsorb water, without the necessity for the time, energy, temperature,mixing, and so forth that might otherwise be required by surface mixing.Thus, the composition in accordance with the invention is immediatelytransportable and flows, relying on the drilling or fracturing fluid asits carrier.

Moreover, as the polymer tends to pick up more water, the density of thegranule of substrate and polymer powder becomes closer to the density ofwater. Accordingly, the size increase and the density change tend todrive the particles of the composition even more homogeneously with theflowing fluid. Thus, the sand does not settle out in various eddies,obstructions, and other locations of low velocity. Rather, the sandcontinues to be carried with the fluid, providing a double benefit. Thatis, the sand weight and area helps to initially mix and drive theparticles (granules) with the fluid. Thereafter, the hydration of thepolymer tends to increase the surface area and reduce the density of thegranule or particle, tending to make the particles flow even better andmore homogeneously with the surrounding fluid.

Ultimately, as the particles (granules) of the composition flow intofracture locations, they provide very small proppants as the substrate,such as sand, becomes trapped and lodged at various choke points.Nevertheless, because of the small size, the sand or other substrateacting as a proppant, simply needs to provide an offset, keepingfractured surfaces from collapsing back against one another. Byproviding the small, strong points of separation, the substrate providesa well distributed proppant, carried to maximum extent that the fluidswill travel, and deposited in various traps, choke points, and the like.

The net saving in time, money, energy for heating and pumping, and thelike is significant. Meanwhile, various technologies for reducingfriction in the flow of fluid pumped into bore holes and other formationspaces is described in several patents, including U.S. Pat. No.3,868,328, issued Feb. 25, 1975 to Boothe et al. and directed tofriction reducing compounds, as well as U.S. Pat. No. 3,768,565, issuedOct. 30, 1973 to Persinski et al. and directed to friction reducing,U.S. Patent Application Publication US 2001/0245114 A1 by Gupta et al.directed to well servicing fluid, and U.S. Patent ApplicationPublication US 2008/0064614 A1 by Ahrenst et al. and directed tofriction reduction fluids, all described various techniques, materials,methods, and apparatus for developing, implementing, and benefiting fromvarious well fluids. All the foregoing patent application publicationsand patents are hereby incorporated by reference.

Similarly, the development of various chemicals has been ubiquitous inoil field development. For example, U.S. Pat. No. 3,442,803, issued May6, 1969 to Hoover et al. is directed to thickened friction reducers,discusses various chemical compositions, and is also incorporated hereinby reference in its entirety.

In one embodiment of an apparatus, composition and method in accordancewith the invention, a method may be used for formation fracturing. Theformation may be in rock and within or near an oil reservoirunderground. One may select an oil field region having a formation to befractured. Fracturing may be sought to increase production. By providinga bore into the formation and a pump, a carrier material, typicallycomprising a liquid, and sometimes other materials dissolved or carriedtherein may be pumped into the formation through the bore.

The carrier as a liquid, or slurry comprising a liquid, or otherwisecontaining a liquid may be driven by the pump to be pressurized into theformation. However, the carrier may be provided an additive formed asgranules. Each granule may include a substrate, such as a grain of sand,ceramic sand, crushed rock, other rock products, or the like havingbonded thereto many particles (e.g. powder) formed from a polymer.

The polymer may be selected to have various properties, includinglubricity, water absorption, water solubility, or the like. Thishydrophilic polymer may be bonded permanently, temporarily, or the liketo secure to the substrate. Various binders may be used alone or incombination. These may range from a solvent (e.g., organic or water)simply softening the polymer itself to bond it, to glues, sugars,molasses, and various other saccharides, as well as other products,including starches, other polymers, and so forth.

Thus, with some bonds, the polymer powder may be less permanent orattached to have a bond that is less robust. Over time, the polymerpowder so attached may wear off, pull away, or otherwise remove from thesubstrate into the carrier fluid, and may even act as a viscous agent,lubricant, or the like in the carrier.

The method may include introducing the additive directly into thecarrier. The more dense substrate will immediately submerge the granulesin the carrier at ambient conditions. Thus heating, extensive mixing,waiting, and the like may be dispensed with, as the granules typicallywill not float or resist mixing once initial surface tension is broken.

Pumping the carrier toward the formation is possible immediately. Thecarrier fluid carries the granules by the liquid dragging against thesubstrate (with the particles of polymer attached. The substrate's crosssectional area engages immediately the surrounding liquid, dragging itinto the carrier to flow substantially immediately therewith.

Meanwhile, weighting, by the substrate of the polymer, permits thegranules to flow into and with the carrier independently from absorptionof any of the liquid into the polymer. Nevertheless, over time,absorbing by the polymer a portion of the liquid results in the polymerexpanding and providing by the polymer, lubricity to the carrier withrespect to the formation;

Creating fractures may be accomplished by pressurizing the carrier inthe formation. This creates fissures or fractures. Thus, flowing of thecarrier and particles throughout the fractures or fissures in theformation results in lodging, by the particles, within those fracturesor fissures. Unable to re-align, adjacent surfaces of rock, now fracturecannot close back together due to propping open the fractures by thesubstrate granules lodging in the fractures.

The substrate is best if selected from an inorganic material, such assand, ceramic sand, or other hard, strong, rock product. The polymer maybe selected from natural or synthetically formulated polymers. Forexample polymers of at acrylic acid, acrylate, and various amides areavailable. Polyacrylamide has been demonstrated suitable for allproperties discussed above.

In fracturing a rock formation, the method may include providing anadditive comprising a substrate formed as granules, each having anexterior surface, particles formed of a hydrophilic material, theparticles being comminuted to a size smaller than the size of thegranules and having first and second sides comprising surfaces. Thegranules may each be coated with the particles, the particles being dryand bonded to the exterior surface by any suitable binder, including thepolymer softened with a solvent. The particles are each secured by thefirst side to the granules, the second side extending radially outwardtherefrom.

Upon identifying a reservoir, typically far underground from thousandsof feet to miles, perhaps, and extending in a formation of rock, oneneeds to provide a bore into the formation. Providing a carrier,comprising a liquid, and possibly other materials known in the art, isfor the purpose of fracturing the formation. Introducing the additivedirectly into the liquid at ambient conditions is possible, because thesubstrate weighs the granules down, and there is no need for longmixing, heating or the like as in addition of polymers directly to thecarrier.

Thus, pumping may continue or begin immediately to move the carrier andadditive down the bore and toward the formation. This results inexposing the second sides of the polymer powder particles directly tothe liquid during transit of the carrier and additive toward and intothe formation. The polymer particles thus begin absorbing, a portion ofthe liquid, typically principally water. Swelling of the polymerincreases the size, effective diameter, and cross-sectional area, thusincreasing the fluid drag on the granules.

Fracturing, typically by hydraulic pressure in the carrier createsfissures in the formation by fracturing the rock pieces in bending, orby layer separation, with tensile stresses breaking the rock. Theresulting fissures allow carrying, by the carrier, of the granules intothe fissures. However, fissures vary in size and path, resulting inlodging of granules, within the fissures. The granules do not settle outfrom the carrier, and thus may travel far into the formation and everyfissure. However, each time a grain or granule is lodged like a chockstone, it obstructs the ability of the adjacent rock surfaces to closeback with one another.

Thus, rather than the proppant (substrate) settling out ineffectually,failing to prop open the fissures, the granules are swept forcefullywith the flow of the carrier wherever the carrier can flow, untillodged. Meanwhile, the lubricity of the polymer aids the granules, andthus the substrate from being slowed, trapped, or settled out by theslow flowing boundary layer at the solid wall bounding the flow.

In summary, weighting, by the substrate, sinks the polymer into thecarrier readily and independently from absorption of the liquid into thepolymer. Mixing, dissolving, and so forth are unnecessary, as thesubstrate drags the polymer into the carrier, and the carrier drags thegranule along with it in its flow path. Lubrication is provided by thepolymer between the substrate of each granule and adjacent solid wallsof the bore, passages previously existing in the formation, and thefissures formed by fracturing. Any separating, by some of the powderedpolymer particles from the substrate, still reduces friction drag onpassage of the carrier and particles within the formation.

A composition for fracturing and propping a formation of rock mayinclude a fluid operating as a carrier to be pumped into a rockformation, a substrate comprising granules of an inorganic material,each granule having an outer surface and a size characterized by amaximum dimension thereacross, and all the granules together having anaverage maximum dimension corresponding thereto. A polymer comprising ahydrophilic material selected to absorb water in an amount greater thanthe weight thereof may be bound to the substrate. The polymer iscomminuted to particles, each particle having a size characterized by amaximum dimension thereacross.

All the polymer particles may be characterized by an average maximumdimension, and an effective (e.g. hydraulic diameter). The averagemaximum dimension of the particles is best if smaller, preferably muchsmaller, than the average maximum dimension of the granules.

The particles of the polymer, bound to the substrate, will travel withit in the fluid. Particles of the polymer are thus further directlyexposed to water in the fluid during travel with the fluid. Thegranules, flowing in the fluid, are carried by the hydrodynamic drag ofthe fluid against the cross-sectional area of the granules coated withthe particles of the polymer. The polymer, selected to expand byabsorbing water directly from the fluid, increases the area and drag,assisting distribution in the formation by the carrier fluid. Thepolymer meanwhile operates as a lubricant lubricating the motion of thesubstrate against the formation during flow of the granules againstsolid surfaces in the formation, bore, and fracture fissures.

The inorganic material, such as sand, ceramic sand, or the like istypically sized to lodge in fissures formed in the formation and hasmechanical properties rendering it a proppant capable of holding openfissures formed in the formation.

In one embodiment, a method for making self-suspending proppantparticles is provided. The method includes mixing a volume of substrateparticles with a liquid binder so that at least a portion of the volumeof substrate particles are at least partly covered with the liquidbinder on an outer surface, thereby forming binder-coated substrateparticles. The method also includes using a water-in-oil emulsion toapply a first water-absorbing polymer to at least a portion of thebinder-coated substrate particles so that the at least a portion of thebinder-coated substrate particles are at least partly coated with thefirst water-absorbing polymer, thereby forming intermediatepolymer-coated substrate particles. The method further includes coatingat least a portion of the intermediate polymer-coated substrateparticles with a second water-absorbing polymer to thereby form polymercoated substrate particles; and drying the polymer coated substrateparticles to remove at least a portion of the water-in-oil emulsion tothereby form self-suspending proppant particles. Each of theself-suspending proppant particles are at least partly coated with thesecond water-absorbing polymer.

In another embodiment, a method for making self-suspending proppantparticles is provided. The method includes adding a volume of substrateparticles to a mixing vessel, the substrate particles including sandparticles. The method also includes applying a first water-absorbingpolymer suspended in a liquid to at least a portion of the substrateparticles in the mixing vessel, so that the at least a portion of thesubstrate particles are at least partly coated with the firstwater-absorbing polymer, thereby forming intermediate polymer-coatedsubstrate particles. The first water-absorbing polymer includes ananionic polyacrylamide. The method further includes coating at least aportion of the intermediate polymer-coated substrate particles with asecond water-absorbing polymer in powdered form to thereby formpolymer-coated substrate particles; and drying at least a portion of thepolymer coated substrate particles to remove at least a portion of theliquid thereby forming self-suspending proppant particles.

In yet another embodiment, self-suspending proppant particles areprovided. The self-suspending proppant particles including substrateparticles, the substrate particles including sand. The self-suspendingproppant particles further including an outer polymeric coatingpositioned on an outer surface of each of the substrate particles, theouter polymeric coating including at least one water-absorbing polymerin powdered form, where at least a portion of the outer polymericcoating is applied to each of the substrate particles using awater-in-oil emulsion.

In another embodiment, a method for making self-suspending proppantparticles is provided. The method includes mixing substrate particleswith a liquid binder so that at least a portion of the substrateparticles are at least partly coated with the liquid binder. The methodalso includes applying one or more water-absorbing polymers to the atleast a portion of the substrate particles that are at least partlycoated with liquid binder to form polymer-coated substrate particles.The applying one or more water-absorbing polymers includes applying afirst water-absorbing polymer in a first form, and applying a secondwater-absorbing polymer in a second form that is different from thefirst form, where the first and second forms are selected from a powderand an emulsion. Each of the first and second water-absorbing polymersis about 20 mol. % anionic to about 50 mo. % anionic. The method furtherincludes exposing the polymer-coated substrate particles to heatsufficient to cause crosslinking in at least a portion of the one ormore water-absorbing polymers present on the polymer coated substrateparticles, thereby forming self-suspending proppant particles.

In yet another embodiment, a method for making self-suspending proppantparticles is provided. The method includes adding a volume of substrateparticles to a mixing vessel, the substrate particles comprising sand.The method also includes coating at least a portion of the substrateparticles with glycerol in the mixing vessel to form binder-coatedsubstrate particles. Further, the method includes mixing a firstwater-absorbing polymer with the binder-coated substrate particles inthe mixing vessel. The first water-absorbing polymer includes aco-polymer of acrylamide monomers and acrylate monomers, where the firstwater-absorbing polymer is about 20 mol. % anionic to about 50 mo. %anionic, and where the first water-absorbing polymer is in powderedform. The method also includes, subsequent to mixing the firstwater-absorbing polymer with the binder-coated substrate particles,mixing a second water-absorbing polymer with first water-absorbingpolymer and the binder-coated substrate particles to form polymer-coatedsubstrate particles. The second water-absorbing polymer including aco-polymer of acrylamide monomers and acrylate monomers, where thesecond water-absorbing polymer is about 20 mol. % anionic to about 50mo. % anionic. The second water-absorbing polymer is present in awater-in-oil emulsion, and each of the polymer-coated substrateparticles includes a polymeric coating that includes the firstwater-absorbing polymer and the second water-absorbing polymer. Themethod further includes exposing the polymer-coated substrate particlesto heat sufficient to cause crosslinking in the polymeric coatingpresent on the polymer-coated substrate particles, thereby formingself-suspending proppant particles.

In yet another embodiment, self-suspending proppant particles areprovided. The self-suspending proppant particles include substrateparticles, the substrate particles including sand. The self-suspendingproppant particles further include an outer polymeric coating positionedon an outer surface of each of the substrate particles, the outerpolymeric coating including first and second water-absorbing polymersthat are at least partly covalently cross-linked. Each of the first andsecond water-absorbing polymers include a co-polymer of acrylatemonomers and acrylamide monomers, where a molecular weight of the firstwater-absorbing polymer is at least about 1 Million Daltons (g/mol)greater than a molecular weight of the second water-absorbing polymer.Each of the first and second water-absorbing polymers is about 20 mol. %to about 50 mol. % anionic, where the first water-absorbing polymer wasapplied to the substrate particles in the form of an emulsion, andwherein the second water-absorbing polymer was applied to the substrateparticles in powdered form. The self-suspending proppant particlesremain suspended in a 1000 ppm CaCO3 aqueous solution for at least 30minutes at a temperature of 170° F.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the present invention will become more fullyapparent from the following description and appended claims, taken inconjunction with the accompanying drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are,therefore, not to be considered limiting of its scope, the inventionwill be described with additional specificity and detail through use ofthe accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of a material including asubstrate provided with a binder securing a hydrating polymer thereto inaccordance with the invention;

FIG. 2 is a schematic block diagram of one embodiment of a process forformulating and producing fluid additive particles in accordance withthe invention;

FIG. 3 is a schematic diagram of the fluid-particle interaction in anapparatus, composition, and method in accordance with the invention;

FIG. 4 is a chart illustrating qualitatively the relationship betweenvolumetric increase over time at various temperatures, illustrating theimproved activation with minimum mixing and temperature increase ofparticles in accordance with the invention;

FIG. 5 is a schematic diagram illustrating one embodiment of frictionreducing by polymers used in compositions in accordance with theinvention;

FIG. 6 is a schematic diagram of the fracturing and proppant action ofparticles in accordance with a method and composition according to theinvention;

FIG. 7 is a schematic block diagram of a fracturing and propping processusing compositions and methods in accordance with the invention,

FIG. 8 is an image of various self-suspending proppants in the presenceof various flowing agents after being exposed to heat and humidity asdescribed in Example 3;

FIG. 9 is an image of the various self-suspending proppants with variousflowing agents of FIG. 9 after being exposed to additional heat andhumidity as described in Example 3;

FIG. 10A depicts various self-suspending proppants with various flowingagents after being centrifuged and inverted for an initial attempt toremove the proppants from a centrifuge tube as described in Example 3;and

FIG. 10B depicts the self-suspending proppants of FIG. 10A after beingremoved from the centrifuge tubes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the drawingsherein, could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the system and method of the present invention, asrepresented in the drawings, is not intended to limit the scope of theinvention, as claimed, but is merely representative of variousembodiments of the invention. The illustrated embodiments of theinvention will be best understood by reference to the drawings, whereinlike parts are designated by like numerals throughout.

Referring to FIG. 1, a material 10 in accordance with the invention mayinclude a substrate 12 formed of a suitable material for placement inthe vicinity of a fracture region. For example, a substrate may be aparticle of sand, ceramic sand, volcanic grit, or other hard material.In some embodiments, a substrate may be formed of organic or inorganicmaterial. Nevertheless, it has been found effective to use sand as asubstrate 12 inasmuch as it is submersible in water and will not floatas many organic materials will when dry. Likewise, the sand as substrate12 is comminuted to such a small size that interstices betweenindividual grains of the sand substrate 12 provide ample space andminimum distance for water to surround each of the substrate 12particles.

In the illustrated embodiment, a binder 14 may be distributed as acomparatively thin layer on the surface of the substrate 12. Typicalmaterials for binders may include both temporary and permanent binders14. Permanent binders include many polymers, natural and synthetic.Temporary binders may be sugar-based or other water soluble materials.For example, corn syrup, molasses, and the like may form temporarybinders. In the presence of water, such material may ultimatelydissolve. Nevertheless, so long as the substrate 12 is not turned,mixed, or otherwise disturbed significantly, any other materialssupported by the binder 14 would not be expected to dislocate.

Otherwise, certain naturally or synthetically occurring polymers mayalso be used as a binder 14. Lignicite may be used as a binder 14.Lignicite is a byproduct of wood, and provides material having goodadhesive properties, and substantial permanence as a binder 14 on asubstrate 12. Any suitable insoluble polymer may be used for morepermanent binding.

Other polymers may be used to form a binder 14. For example, variousmaterials used as glues, including mucilage, gelatin, other watersoluble polymers including, for example, Elmer's™ glue, and the like mayalso operate as binders 14 to bind materials to a substrate 12.

In certain embodiments, the substrate 12 may be used in oil fields as asubstrate 12 for polymer additives to fracture fluids. In othersituations, the substrate 12 may be implemented as a proppant.

Pigment 16 may be implemented in any of several manners. For example,the substrate 12 may have pigment 16 applied prior to the application ofthe binder 14. In alternative embodiments, the pigment 16 may actuallybe included in the binder 14, which becomes a pigmented coating on thesubstrate 12. In yet other embodiments, the pigments 16 may be added toa hydration particle 18 either as a pigment 16 mixed therein, or as apigment 16 applied as a coating thereto. Thus the location of thepigment 16 in the Figures is schematic and may take alternative locationor application method.

Particles 18 of a hydrophilic polymer material may be bonded to thesubstrate 12 by the binder 14. Particles may be sized to substantiallycoat or periodically coat the substrate 12.

In certain embodiments, the hydrophilic material 18 may be a powderedpolymeric material 18 such as polyacrylamide or any of the materials inthe patent documents incorporated by reference. In other embodiments,the particles 18 may actually be organic material having capillaryaction to readily absorb and hold water. In one presently contemplatedembodiment of an apparatus in accordance with the invention, theparticles 18 may be powdered polymeric material in a dehydrated state,and having a capacity to absorb water, typically many times the weight(e.g., five to forty times) of a particular particle 18.

The substrate 12, in certain embodiments, may be some form of sand orgrannular material. The sand will typically be cleaned and washed toremove dust and organic material that may inhibit the binder 14 frombeing effective. Likewise, the substrate 12 may be sized of any suitablesize. For example, sand particles may range from much less than amillimeter in effective diameter or distance thereacross toapproximately two millimeters across. Very coarse sands or ceramic sandsmay have even larger effective diameters. Hydraulic diameter iseffective diameter (four times the area divided by the wettedperimeter). However, in one presently contemplated embodiment, washedand dried sand such as is used in construction, such as in concrete, hasbeen found to be suitable. Fine sands such as masonry sands tend to besmaller, and also can function suitably in accordance with theinvention.

Accordingly, the distance across each powder particle 18 may be selectedto provide an effective coating of powdered particles 18 on thesubstrate 12. In one presently contemplated embodiment, the effectivediameter of the particles 18 may be from about a 30 mesh size to about a100 mesh size. For example, a sieve system for classifying particles hasvarious mesh sizes. A size of about 30 mesh, able to pass through a 30mesh sieve, (i.e., about 0.6 mm) has been found suitable. Likewise,powdering the particles 18 to a size sufficiently small to pass througha 100 mesh (i.e., about 0.015 mm) sieve is also satisfactory. A meshsize of from about 50 mesh to about 75 mesh is an appropriate materialto obtain excellent adhesion of particles 18 in the binder 14, with asuitable size of the particles 18 to absorb significant liquid at thesurface of the substrate 12.

As a practical matter, about half the volume of a container containing asubstrate 12 as particulate matter will be space, interstices betweenthe granules of the substrate 12. One advantage of using materials suchas sand as the substrate 12 is that a coating of the particles 18 mayprovide a substantial volume of water once the particles 18 are fullysaturated. By contrast, where the size of the particles 18 is too manyorders of magnitude smaller than the effective diameter or size of thesubstrate particles 12, less of the space between the substrateparticles 12 is effectively used for storing water. Thus, sand as asubstrate 12 coated by particles 18 of a hydrophilic material such as apolymer will provide substantial space between the substrate particles12 to hold water-laden particles 18.

The diameter of the particles 18, or the effective diameter thereof, istypically within about an order of magnitude (e.g., 10.times.) smallerthan the effective diameter of the particles of the substrate 12. Thisorder of magnitude may be changed. For example, the order of magnitudedifference less than about 1 order of magnitude (i.e., 10.times.) maystill be effective. Similarly, an order of magnitude difference of 2(i.e., 100.times.) may also function.

However, with particles 18 too much smaller than an order of magnitudesmaller than the effective diameter of the substrate 12, theinterstitial space may not be as effectively used. Likewise, with aneffective diameter of particles 18 near or larger than about 1 order ofmagnitude smaller than the size of the particles of the substrate 12,binding may be less effective and the particles 18 may interfere morewith the substrate itself as well as the flow of water through theinterstitial spaces needed in order to properly hydrate a material 10.

Referring to FIG. 2, an embodiment of a process for formulating thematerial 10 may involve cleaning 22 the material of the substrate 12.Likewise, the material of the substrate 12 may be dried 24 to make itmore effective in receiving a binder 14. The material of the substrate12 may then be blended 26.

One embodiment, a ribbon blender provides an effective mechanism toperform continuous blending as the binder 14 is added 28. Other types ofmixers, such as rotary mixers, and the like may be used. However, aribbon blender provides a blending 26 that is effective to distributebinder 14 as it is added 28.

For example, if an individual particle of the substrate 12 receives toomuch binder 14, and thus begins to agglomerate with other particles ofthe substrate 12, a ribbon binder will tend to separate the particles asa natural consequences of its shearing and drawing action duringblending 26.

As the binder 14 is added 28 to the mixture being blended 26, theindividual particles of the substrate 12 will be substantially evenlycoated. At this stage, the binder 14 may also be heated in order toreduce its viscosity and improve blending. Likewise, the material of thesubstrate 12 or the environment of the blending 26 may be heated inorder to improve the evenness of the distribution of the binder 14 onthe surfaces of the substrate 12 materials or particles 12.

Blending 26 of the binder 14 into the material of the substrate 12 iscomplete when coating is substantially even, and the texture of thematerial 10 has an ability to clump, yet is easily crumbled and brokeninto individual particles. At that point, addition 30 of the hydrophilicparticles 18 may be accomplished.

For example, adding 30 the particles 18 as a powder into the blending 26is a naturally stable process. Typically the particles 18 attach to thebinder 14 of the substrate 12 particles, thus removing from activitythat location. Accordingly, other particles 18 rather than agglomeratingwith their own type of material will continue to tumble in the blending26 until exposed to a suitable location of binder 14 of the substrate12. Thus, the adding 30 of the particles 18 or powder 18 of hydrophilicmaterial will tend to be a naturally stable process providing asubstantially even coating on all the particles of the substrate 12.

Just as marshmallows are dusted with corn starch, rendering them nolonger tacky with respect to one another, the material 10 formulated bythe process 20 are dusted with particles 18 and will pour freely.Accordingly, distribution 32 may be conducted in a variety of ways andmay include one or several processes. For example, distribution mayinclude marketing distribution from packaging after completion ofblending 26, shipping to distributers and retailers, and purchase andapplication by users.

An important part of distribution 32 is the deployment of the material10. In one embodiment of an apparatus and method in accordance with theinvention, the material 10 may be poured, as if it were simply sand 12or other substrate 12 alone. Since the powder 18 or particles 18 havesubstantially occupied the binder 14, the material 10 will not bind toitself, but will readily pour as the initial substrate material 12 will.

The material 10 may typically include from about 1 percent to about 20percent of a hydrophilic material 18 or particles 18. The particles 18may be formed of a naturally occurring material, such as a cellulose,gelatin, organic material, or the like.

In one embodiment, a synthetic gel, such as polyacrylamide may be usedfor the particles 18, in a ratio of from about 1 to about 20 percentparticles 18 compared to the weight of the substrate 12. In experiments,a range of from about 5 to about 10 percent has been found to be themost effective for the amount particles 18.

Sizes of particles 18 may range from about 20 mesh to smaller than 100mesh. Particles 18 of from about 50 to about 75 mesh have been foundmost effective.

The binder 14 may typically be in the range of from about in ¼ percentto about 3 percent of the weight of the substrate 12. A range of fromabout ¾ percent to about 11/2 percent has been found to work best. Thatis, with a binder such as lignicite, ¼ of 1 percent has been found notto provide as reliable binding of particles 18 to the substrate 12.Meanwhile, a ratio of higher than about 3 percent by weight of binder 14to the amount of a substrate 12, such as sand, when using lignicite asthe binder 14, tends to provide too much agglomeration. The pouringability of the material 10 is inhibited as well as the blending 26, dueto agglomeration. Other binders also operate, including several smallermolecules that are water soluble. For example, glues, gelatins, sugars,molasses, and the like may be used as a binder 14. Insoluble binders arealso useful and more permanent.

One substantial advantage for the material 10 in accordance with thepresent invention is that the material remains flowable as a sand-likematerial 10 into the fluids to be used in oil field fracturing. Thus,handling and application is simple, and the ability of granular material10 to flow under and around small interstices of fractures provides fora very effective application.

Referring to FIG. 3, a formation 80 such as a reservoir area of an oilmay increase large and small flows 82 in passages 84 formed in the rock86 of the formation 80. Typically, the flow 82 represented by arrows 82indicating the development of flow at a faster speed in center of apassage 84, and the lower velocity near the wall 88 of the passage 84,illustrates the flow 82 of fluid in the passage 84.

In the illustrated embodiment, the granules 10 or large compositeparticles 10 or the materials 10 formed as a granulated material 10,having the substrate 12 in the center column with the polymer 18 adheredby a binder 12 on the outside thereof. This material 10 may be added toa flow 82 being pumped into a formation 80. Initially, a particle 10will have an effective diameter 90 a. In this condition, the particle 10of material 10 is largely dependant on the density of the substrate 12,which constitutes the majority of its volume. Eventually, over time,with exposure to the liquid 82 or flow 82 and the water of that flow 82,the polymer 18 will absorb water, increasing in its effective diameter90 b. Ultimately, the polymer 18 or the polymer powder 18 willeventually become fully hydrated, increasing many times its size, andbeginning to dominate the effective diameter 90 c or hydraulic diameter90 c of the particle 10.

Initially, the diameter 90 a reflects the comparatively smaller size andlarger density of the particle 10 dominated by the weigh of thesubstrate 12, such as sand, ceramic sand, or some other hard and strongmaterial. Ultimately, the diameter 90 a or effective diameter 90 a issufficient to provide fluid drag according to fluid dynamic equations,drawing the particle 10 into the flow 82.

Meanwhile, the increase in diameter 90 b and the ultimate effectivediameter 90 c result in reduction of the density of the particle 10 asthe polymer 18 absorbs more water, bringing the net density of theparticle 10 closer to the density of water. Accordingly, the particles10 flow with the water exactly in sync, so to speak, rather thansettling out as a bare substrate 12 would do.

For example, in areas where eddies in the flow occur, such as corners,crevices, walls, and the like, heavy materials having higher density,such as sand and the like, normally will tend to drift out of the flow,toward a wall 88, and ultimately will settle out. Instead, by virtue ofthe large “sail” presented by the larger diameter 90 c of a fullyhydrated polymer 18, each particle 10 stays with the flow 82 in passage84, providing much more effective transport.

Referring to FIG. 4, a chart 92 illustrates a volume axis 94representing the volume of a particle 10 or material 10 in accordancewith the invention. The volume axis 90 is displayed orthogonally withrespect to a time axis 96, representing the passage of time of theparticle 10 submerged in a carrier 82 or flow 82 of fluid 82. Typically,at different temperatures, illustrated by curves 98 a-98 e, with thetemperature associated with curve 98 a being the coldest and thetemperature associated with the curve 98 e being the hottest, one canvisualize how heat added to a fluid flow 82 tends to increase thechemical activity and thus the rate of absorption of water into apolymer 18.

In an apparatus and method in accordance with the invention, theparticles 10 may be added directly to a flow 82, without waiting for anysignificant time to absorb water into the polymer 18. Instead, thenormal flow 82 will draw the particles 10 along in a passage 84 whileexposing each individual particle 10 to surrounding fluid 82, thuspromoting maximum rates of exposure and increased rates of absorption.Accordingly, the volume 94 increases, representing an increase in theabsorption of water into the polymer 18.

In an apparatus and method in accordance with the invention, the curve98 a is suitable because the entire travel within the well bore, andwithin the formation 80 by the fluid 82 bearing the particles 10 ispermissible and available as absorption time. By contrast, prior artsystems rely on the increased temperature of curve 98 e in order toprovide the time, temperature, and mixing to work polymers into a flow82 or liquid carrier 82.

Referring to FIG. 5, in one embodiment of an apparatus, composition, andmethod in accordance with the invention, some of the polymer 18 mayeventually be scraped, sheared, or otherwise removed from the particles10. If bonded only by itself with a water solvent, such a separation maybe easier than if bonded by a more durable polymer. Such a release mayeven be engineered, timed, controlled by a solvent, or the like.

Thus, a certain amount of the polymer 18 may be released from thegranule 10 into the carrier fluid 82 to flow with the fluid 82 andoperate as a general friction reducer or provide its other inherentproperties to the carrier fluid 82. By an engineered process of bondingand un-bonding, the polymer powder may be less permanent or attached tohave a bond that is less robust. Over time, the polymer powder soattached may release, tear, wear off, pull away, or otherwise removefrom the substrate into the carrier fluid to act as a viscosity agent,surfactant, lubricant, or the like in the carrier, according to itsknown properties available for modifying the carrier 82.

For example, a polymer 100 or polymer chain 100 may be captured on acorner 102 defining a passage 84 into which a flow 82 will proceed.Accordingly, the corner 102 renders less of an orifice on the passage 84against entry of the flow 82 by virtue of the friction reduction of thepolymer 100 in the fluid, deposited temporarily or permanently about acorner 102. Thus, other particles 10 passing the corner 100 may shearoff a portion of the polymer 18 carried thereby or may rely on thepresence of the polymer 18 as a direct friction reducing agent on theparticle 10 (granule) itself, permitting the particles 10 to pass moreeasily with the flow 82 into the passage 84.

Referring to FIG. 6, the fracture process is described in variousliterature, including U.S. Patent Application publication US2009/0065253 by Suarez-Rivera et al. incorporated herein by reference.In a fracturing process, the pressure applied to a formation 80 tends toforce apart large expanses of rock. As a result of that expansion ofpassages 84 in a rock formation 80, the rock is stressed. Pressurepumped into the fluid 82 flowing in the passages 84 within the formation80 results in bending stresses, tensile stresses, and so forth in theformation 80.

In FIG. 6, the forces 110 illustrated the effect of a large pressureapplied over a large area. Since pressure multiplied by area equalsforce, applying an elevated hydraulic pressure to a large surface of arock 86 or rock segment 86 within a formation 80 results in tensileforces. Compressive forces will not tend to break rock. However, atensile force, which may be induced by bending, expansion, or the like,results in fracture of the rock. The fracture of the rock 86 thusresults in condition shown in the lower view, in which the passages 84are mere fissures within the rock 86.

The inset of FIG. 6 magnifies the fissures 84 or passages 84 formed inthe rock 86 and immediately entered by the working fluid 82 being usedfor the fracture. Having the particles 10 formed around substrates 12,the fluid 82 extends into each of the fissures formed. Fissures 84 aresimply passages 84. Some may be large, others small. Proppants 10trapped in a small location may still maintain opened in another openingmuch larger elsewhere on the rock 86. They may also collect and filllarger spaces, eliminating the ability for rocks 86 to return to formerpositions.

After fracturing rock 86 to form all of the fissures 84, the fluid 82will pass through the fissures, carrying particles 10, which eventuallycollect in cavities or reach choke points. In the absence of theparticles 10, fissures 84 could close back up after the fracturing waterleaves. However, by containing the particles 10, the individualsubstrates 12 are themselves rock in the form of sand, ceramic sand, orthe like. Thus, a particle 10 need only obstruct the ability of thefissure 84 to close, and it may “prop” open the fissures 84 precludingthe rock 86 or the pieces of rock 86 from settling back into alignmentwith one another.

Thus, the particles 10 both alone and in collected piles act asproppants left behind by the fluid flow 82, by virtue of the particles10B captured. As a practical matter, it is only the substrate 12 thatacts as a proppant. The polymer 18 may eventually be worn off but caneasily be compressed, distorted, or cut. Regardless, as the fissures 84open, they are back filled and close in at choke points and settlingpoints collecting the substrate 12.

Referring to FIG. 7, a process 10 may include preparing 112 a fluid 82.Processing 114 other additives other than the particles 10 may be doneaccording to any suitable methods, including prior art processes. Adding116 directly to the fluid 82, the particles 10 as described hereinabove,may be done in such a manner that the operators need not wait forabsorption or any other processes to take place. Additional energy forelevating temperature is not required, neither mixing or the like, otherthan adding 116 directly particles 10 in to the flow 82. The flow 82will immediately grab the particles 10 according the principles of fluiddynamics in which fluid drag is dependent upon a shape factor of theparticle 10, the density of the fluid 82, the square of velocity of thefluid, and so forth, as defined in engineering fluid mechanics.

The fluid 82 now bearing the particles 10 would be immediately pumped118 into the formation 80 that is the reservoir 80 of 8 and oil field.Eventually, pressurizing 120 the reservoir by pressurizing the fluid 82results in creating 122 fractures 84 or fissures 84 within the formation80 by breaking up the rock 86 of the formation 80. A fracture 84 withenough displacement may make a site for material 10 to stagnate andcollect.

Creating 122 fracture lines throughout the formation 80 is followed bypenetrating 124, by the particles 10 borne in the fluid 82 into thepassages 84 or fissures 84 in the rock 86 of the formation 80. Wheneverthe flow 82 of fluid 82 carries a particle 10 to a choke point 108 in apassage 84, as illustrated in FIG. 6, a particle 10 will be lodged asillustrated in the inset of FIG. 6, a particle 10 with its polymer 18still secured and intact may be lodged. Similarly, the substrate 12 maybe lodged 126 and the polymer 18 may stripped therefrom by theconsequent or subsequent flowing of material in the flow 82. Likewise,piles of stagnant particles 10 may backfill spaces, precluding rock 86settling back in.

After the lodging 126 or propping 126 of the fissures 84 by thesubstrate 12, in the particles 10, the passages 84 will remain open.These fissures 84 may then be used to later withdraw 128 the fluid 82from the formation 80. Thereafter, returning 130 the formation 80 toproduction may occur in any manner suitable. For example, heat may beadded to the formation, liquid may be run through the formation as adriver to push petroleum out, or the like.

As discussed above, the material 10 may be utilized as self-suspendingproppants in hydraulic fracturing. As used herein, the material 10 mayalso be referred to as self-suspending proppants.

Methods for Forming Self-Suspending Proppant Particles

In various embodiments, as discussed above, the self-suspendingproppants described herein may include a substrate, e.g., sand, havingan outer polymeric coating that is water-absorbing. In such embodiments,a water-absorbing outer polymeric coating comprises one or morewater-absorbing polymers, such as the water-absorbing polymers discussedbelow. In addition, in various embodiments, as discussed above, theself-suspending proppants may include a binder, which may aid insecuring at least a portion of the water-absorbing polymeric coating tothe substrate.

In certain embodiments, the substrate can include frac sand. In one ormore embodiments, the frac sand can be graded as a 12-20 mesh frac sand,a 16-30 mesh frac sand, a 20-40 mesh frac sand, a 30-50 mesh frac sand,a 30-70 mesh frac sand, a 40-70 mesh frac sand, or a 100+mesh frac sand.In certain embodiments, a combination of various grades of frac sand canbe used, such as a combination of the grades listed above. Such fracsands and grades are commercially available.

Methods for Forming Self-Suspending Proppant Particles: Binder-CoatedSubstrate Particles

In embodiments, as discussed above, the self-suspending proppants can bemade by first mixing a substrate with a binder to form binder-coatedsubstrate particles. In certain embodiments, the binder may be a liquidbinder, such as glycerol. Other liquid binders are described above.

In one or more embodiments, when mixing a binder with a substrate, onemay add at least about 0.05 wt. % binder, at least about 0.1 wt. %binder, at least about 0.15 wt. % binder, or at least about 0.2 wt. %binder; and/or less than about 3 wt. % binder, less than about 2 wt. %binder, less than about 1 wt. % binder, or less than about 0.5 wt. %binder. In the same or alternative embodiments, when mixing a binderwith a substrate, one may add about 0.25 wt. % binder. As used herein,“wt. % binder” refers to the ratio of the weight of the binder to theweight of the substrate, multiplied by 100.

In embodiments, the binder and substrate can be mixed using anycommercially available mixing device or mixing vessel and a particularone can be chosen by one skilled in the art for a specific purpose. Incertain embodiments, a ribbon mixer or paddle mixer may be utilized whenmixing the binder and substrate.

In certain embodiments, the binder and substrate may be mixed for a timesufficient to substantially evenly apply the binder to the substrateparticles. In one or more embodiments, the binder may be mixed with thesubstrate for at least about 1-2 minutes. In certain embodiments, uponmixing the binder and substrate under the conditions described herein, asubstantially portion, or substantially all of, the binder may be coatedonto the outer surface of the substrate particles so that there islittle to no excess free binder.

Methods for Forming Self-Suspending Proppant Particles: IntermediatePolymer-Coated Substrate Particles

In embodiments, at least a portion of the binder-coated substrateparticles, or substrate particles without binder, may be coated with oneor more polymeric materials, such as water-absorbing polymers to formintermediate polymer-coated substrate particles. In one or moreembodiments, the water-absorbing polymers utilized to form theintermediate polymer-coated substrate particles can includepolyacrylamide, polyacrylates, or a combination thereof. In certainembodiments, the water absorbing polymers can include a co-polymer ofacrylamide monomers and acrylate monomers. In such embodiments, theco-polymer of acrylamide monomers and acrylate monomers may be a randomco-polymer of acrylate monomers and acrylamide monomers such that theacrylate monomers and the acrylamide monomers are randomly positionedwithin the co-polymer.

In embodiments, any of the water-absorbing polymers disclosed herein maybe linear or cross-linked. As used herein, a polymer that is linearrefers to a polymer that is not cross-linked with itself or anotherpolymer by covalent bonds and/or ionic bonds. In certain embodiments, apolymer that is linear, while not being cross-linked, can include simplelinear polymers and branched linear polymers. A simple linear polymerrefers to a polymer having a single long chain, while a branched linearpolymer refers to a polymer having a long chain with one or more shorterchains branched off from the long chain.

In certain embodiments, the water-absorbing polymer can include ananionic polymer, such as polyacrylamide or a co-polymer of acrylatemonomers and acrylamide monomers. As discussed above, the polyacrylamidecan include a linear polyacrylamide. In various embodiments, the anionicpolyacrylamide or other anionic polymer, such as an anionic co-polymerof acrylamide monomers and acrylate monomers, can have an anionic chargein an amount of about 10 mol %, about 20 mol %, about 30 mol %, about 40mol % or about 50 mol %. In embodiments, the anionic polymer orco-polymer can be about 10 mol % to about 50 mol % anionic, about 20 mol% to about 50 mol % anionic, or about 30 mol % of about 40 mol %anionic. In certain embodiments, within an anionic co-polymer ofacrylate monomers and acrylamide monomers, the anionic content maysubstantially correlate to the content of the acrylate monomers in theco-polymer. For instance, in such embodiments, a co-polymer of acrylatemonomers and acrylamide monomers that is about 20 mol % to 50 mol %anionic, can include about 20 mol % to about 50 mol % acrylate monomers.

In various embodiments, a co-polymer of acrylate monomers and acrylamidemonomers may include neutral or non-ionic acrylamide monomers andanionic acrylate monomers. In such embodiments, the acrylamide monomerscan be present in the co-polymer in an amount of at least about 50 mol%, at least about 60 mol %, at least about 70 mol %, or at least about90 mol %; or from about 40 mol % to about 90 mol %, or from about 50 mol% to about 90 mol %, or from about 50 mol % to about 80 mol %. In thesame or alternative embodiments, the water-absorbing polymer can includea non-ionic polymer, cationic polymer, anionic polymer, or a combinationthereof. It is appreciated that one skilled in the art is aware thatsuch types of water-absorbing polymers can be commercially obtained.Representative commercial vendors includes SNF and Evonik.

In certain embodiments, utilizing water-absorbing polymers having theanionic content ranges described above can provide some advantageousfunctional properties to the self-suspending proppants. For example, incertain embodiments, utilizing water-absorbing polymers having theanionic content ranges described above can result in the effectivesuspension of the self-suspending proppants in an aqueous fluid,including brackish water. Further, in certain embodiments, the anioniccontent of a water-absorbing polymer described herein may affect themolecular weight or size of the polymer, which may affect the ability ofthe polymer to form a polymer coating on the substrate particle. In suchan embodiment, utilizing water-absorbing polymers having the anioniccontent ranges described above allows for the appropriate molecularweight of the water-absorbing polymer so that such a polymer can form aneffective polymer coating on the substrate particles. These and otherproperties are discussed further below.

In embodiments, the water-absorbing polymer can have a molecular weightof at least about 1 million Daltons (g/mol), at least about 5 millionDaltons (g/mol), at least about 15 million Daltons (g/mol), or at leastabout 20 million Daltons (g/mol). In the same or alternativeembodiments, the water-absorbing polymer can have a molecular weight ofabout 40 million Daltons (g/mol) or less, or about 30 million Daltons(g/mol) or less.

In certain embodiments, coating the binder-coated substrate particleswith one or more polymeric materials to form intermediate polymer-coatedsubstrate particles can include applying one or more polymeric materialsto a mixing vessel or device containing binder-coated substrateparticles, such as the mixing devices discussed above.

In certain embodiments, the water-absorbing polymers may be suspendingin liquid prior to being added to a mixing vessel containing thesubstrate particles or binder-coated substrate particles. In one or moreembodiments, the water-absorbing polymer may be present in awater-in-oil emulsion (i.e., an invert emulsion). In such embodiments,the water-in-oil emulsion may include the water-absorbing polymer in thewater or aqueous phase, which is emulsified in the larger oil phase,such as a mineral oil or other distillates of petroleum. In addition, insuch embodiments, the water-absorbing polymer may be present in thewater-in-oil emulsion in an amount of about 15 wt. %, about 20 wt. %,about 25 wt. %, about 30 wt. %., about 40 wt. %, or about 50 wt. %. Itis appreciated that one skilled in the art understands that water-in-oilemulsions comprising water-absorbing polymers can be commerciallyobtained. One such commercial vendor is SNF.

In one or more embodiments, when a water-in-oil emulsion is utilized,the emulsion can be added to the binder-coated substrate, or plainsubstrate, in an amount of at least about 0.1 wt. % emulsion relative tothe weight of the substrate, at least about 0.2 wt. %, at least about0.3 wt. %, at least about 0.4 wt. %, at least about 0.5 wt. %, at leastabout 1 wt. %, at least about 2 wt. %, at least about 2.5 wt. %, atleast about 3 wt. %, at least about 4 wt. %, at least about 5 wt. %, orat least about 7.5 wt. %. In the same or alternative embodiments, theemulsion can be added to the binder-coated substrate, or plainsubstrate, in an amount of less than about 20 wt. % emulsion relative tothe weight of the substrate, less than about 15 wt. %, or less thanabout 10 wt. %. As used herein, “wt. % emulsion relative to the weightof the substrate” refers to the ratio of the weight of the emulsion tothe weight of the substrate, multiplied by100.

In certain embodiments, the water-absorbing polymer may be mixed withthe binder-coated substrate particles, or substrate particles, for atime sufficient to substantially evenly apply the water-absorbingpolymer, or the liquid or emulsion containing the water-absorbingpolymer, to the substrate particles to form the intermediatepolymer-coated substrate particles. In one or more embodiments, theintermediate polymer-coated substrate particles can be fully coated withwater-absorbing polymer and/or the liquid or emulsion containing thewater absorbing polymer. In one or more embodiments, the water absorbingpolymer may be mixed with the substrate for about 1-0 minutes, or about2-3 minutes.

Without being bound by any particular theories, in certain embodiments,it is believed that the humectant properties of the binder, such as thehumectant properties of glycerol, may attract the aqueous dispersion ofwater-absorbing polymer from the water-in-oil emulsion to the surface ofthe substrate particles resulting in the coating of the water-absorbingpolymer onto the surface of the substrate particle. In such embodiments,the binder can include other humectants or hygroscopic compounds knownto one skilled in the art.

In alternative embodiments, the water-absorbing polymers may be in driedpowdered form prior to being added to a mixing vessel containing thesubstrate particles or binder-coated substrate particles. The driedpowdered form of the water-absorbing polymers can include any or all ofthe features of the dried powdered form of water-absorbing polymersdiscussed below. For instance, the water-absorbing polymer in driedpowdered form can include powder particles of less than about 300microns, less than about 200 microns, or less than about 100 microns. Inone or more embodiments, the water-absorbing polymer in dried powderedform can include powder particles with a maximum dimension of less thanabout 300 microns, less than about 200 microns, or less than about 100microns. In the same or alternative embodiments, a water-absorbingpolymer in powdered form refers to a dry powdered polymer having a drycontent of: at least about 50%, at least about 75%, at least about 85%,at least about 95%, or at least about 99%; or a dry content of fromabout 75%-100% or about 88%-100%.

In embodiments when the dried powdered form of the water-absorbingpolymer is added to a mixing vessel containing the substrate particlesor binder-coated substrate particles to form intermediate-coatedsubstrate particles, the water-absorbing polymer can be added in anamount of 0.01 wt. % water-absorbing polymer relative to the weight ofthe substrate, at least about 0.1 wt. %, at least about 0.25 wt. %, atleast about 0.5 wt. %, at least about 1 wt. %, at least about 1.5 wt. %,or at least about 2 wt. %; and/or less than about 10 wt. %, less thanabout 7.5 wt. %, less than about 5 wt. %, or less than about 3 wt. %.

Methods for Forming Self-Suspending Proppant Particles: Polymer-CoatedSubstrate Particles

In various embodiments, the intermediate polymer-coated substrateparticles can be coated with a second water-absorbing polymer to formpolymer-coated substrate particles. In certain embodiments, the secondwater absorbing polymer can have any or all of the parameters andproperties of the water-absorbing polymers described above. In variousembodiments, the water-absorbing polymer applied to the intermediatepolymer-coated substrate particles can include an acrylamide andacrylate co-polymer that is cross-linked. In various other embodiments,the water-absorbing polymer applied to the intermediate polymer-coatedsubstrate can include a linear co-polymer of acrylate monomers andacrylamide monomers. In the same or alternative embodiments, thewater-absorbing polymer applied to the intermediate polymer-coatedsubstrate particles can include a linear polyacrylamide. In suchembodiments, the linear polyacrylamide applied at this step may have anincreased molecular weight relative to the water-absorbing polymerapplied in forming the intermediate polymer-coated substrate particles.

In certain embodiments, a water-absorbing polymer utilized in anemulsion can have a greater molecular weight than a water-absorbingpolymer utilized in powdered form. In such embodiments, the molecularweight of the water-absorbing polymer present in an emulsion can be atleast about 1 million Daltons (g/mol), at least about 2 million Daltons(g/mol), or at least about 3 million Daltons (g/mol) greater than themolecular weight of the water-absorbing polymer present in powderedform.

In one or more embodiments, the polymer coated substrate particles canbe formed by applying the second water-absorbing polymer to theintermediate polymer-coated substrate particles in a mixing vessel, suchas the mixing vessels discussed above.

In certain embodiments, the second water-absorbing polymer may bepresent in powdered form when applying to the intermediatepolymer-coated substrate particles. In certain embodiments, awater-absorbing polymer in powdered form refers to a dry powderedpolymer with powder particles of less than about 300 microns, less thanabout 200 microns, or less than about 100 microns. In the same oralternative embodiments, a water-absorbing polymer in powdered formrefers to a dry powdered polymer having a dry content of: at least about50%, at least about 75%, at least about 85%, at least about 95%, or atleast about 99%; or a dry content of from about 75%-100% or about88%-100%. In embodiments, the processes described above for forming theintermediate polymer-coated substrate particles may result in theintermediate polymer-coated substrate particles having an outer wetsurface, due to the application of the binder and/or the application ofa water absorbing polymer in a liquid, such as an emulsion describedabove. In such embodiments, a dry powdered second water-absorbingpolymer may adhere to the wet outer surface of the intermediatepolymer-coated substrate particles.

In one or more embodiments, the second water-absorbing polymer may beapplied in an amount so as to not completely, or discontinuously, coveror coat the outer surface of the intermediate polymer-coated substrateparticles. In alternative embodiments, the second water-absorbingpolymer may be applied in an amount so as to substantially coat or coverthe outer surface of the intermediate polymer-coated substrate.

In certain embodiments, the second water-absorbing polymer may beapplied in an amount of at least about 0.01 wt. % water-absorbingpolymer relative to the weight of the substrate, at least about 0.1 wt.%, at least about 0.25 wt. %, at least about 0.5 wt. %, at least about 1wt. %, at least about 1.5 wt. %, or at least about 2 wt. %; and/or lessthan about 10 wt. %, less than about 7.5 wt. %, less than about 5 wt. %,or less than about 3 wt. %. As used herein, “wt. % water-absorbingpolymer relative to the weight of the substrate” refers to the ratio ofthe weight of the second water-absorbing polymer to the weight of thesubstrate, multiplied by 100.

In embodiments, the second water-absorbing polymer may be mixed with theintermediate polymer-coated substrate particles to form polymer-coatedsubstrate particles for a time sufficient to evenly distribute thesecond water-absorbing polymer amongst the volume of intermediatepolymer-coated substrate particles. In certain embodiments, the secondwater-absorbing polymer may be mixed with the intermediatepolymer-coated substrate particles for less than about 5 minutes, lessthan about 4 minutes, less than about 3 minutes, less than about 2minutes, or less than about 1 minute.

In certain embodiments, the second water-absorbing polymer applied tothe binder-coated substrate particles to form intermediate-coatedsubstrate particles in this step can be present in a liquid, such as anemulsion. In such embodiments, this water-absorbing polymer applied in aliquid can include any or all of the properties of the water-in-oilemulsions discussed above. Further, in such embodiments, thiswater-absorbing polymer in liquid form can be applied to thebinder-coated substrate particles to form intermediate-coated substrateparticles in this step in the amount of at least about 0.1 wt. %emulsion relative to the weight of the substrate, at least about 0.2 wt.%, at least about 0.3 wt. %, at least about 0.4 wt. %, at least about0.5 wt. %, at least about 1 wt. %, at least about 2 wt. %, at leastabout 2.5 wt. %, at least about 3 wt. %, at least about 4 wt. %, atleast about 5 wt. %, or at least about 7.5 wt. %.

As discussed above, the methods described herein can include twoseparate, distinct applications of a water-absorbing polymer indifferent forms from one another, such as powdered form and in the formof an emulsion. In certain aspects, it may be desirable to add thewater-absorbing polymer in powdered form prior to applying thewater-absorbing polymer in the form of an emulsion, since thisparticular order of application can prevent or reduce thewater-absorbing polymer in powdered from from clumping.

Methods for Forming Self-Suspending Proppant Particles: Optional ThirdApplication of Water-Absorbing Polymer

In one or more embodiments, a third application of water-absorbingpolymer may be optionally applied to the polymer-coated substrateparticles. In such embodiments, this optional application of thewater-absorbing polymer can include any or all of the properties andparameters discussed above in applying the water-absorbing polymer tothe binder-coated substrate particles to form the intermediatepolymer-coated substrate particles. For example, in various embodiments,this third application of water-absorbing polymer can include mixing awater-in-oil emulsion comprising a water-soluble polymer with thepolymer-coated substrates in a mixing vessel for 2-3 minutes.

Methods for Forming Self-Suspending Proppant Particles: Drying thePolymer-Coated Substrate

In embodiments, the polymer-coated substrate, with or without the thirdapplication of water-absorbing polymer, can be dried in order to formself-suspending proppant particles. In such embodiments, this dryingstep can remove at least a portion of the liquid with which thewater-absorbing polymer was suspended during application of thewater-absorbing polymer to the substrate, such as that described abovewith reference to the formation of intermediate polymer-coated substrateparticles or the polymer-coated substrate particles.

In certain embodiments, the polymer-coated substrate can be exposed tomild heat, such as a temperature of at least about 100° F., or at leastabout 150° F.; and/or less than about 300° F., less than about 250° F.,or less than about 225° F. In embodiments, the drying temperature can beabout 200° F. or less than about 230° F. Any type of oven or dryingsystem, e.g., fluid bed dryer, can be used to dry the polymer-coatedsubstrate, and a particular system can be chosen by one skilled in theart for a specific purpose. The polymer-coated substrate can be exposedto mild heat for at least about 10 minutes, at least about 20 minutes,or at least about 30 minutes.

In certain embodiments, exposing the polymer-coated substrate particlesto the temperature ranges described above can be sufficient to causecross-linking in at least a portion of the one or more water-absorbingpolymers present on the substrate surface. In such embodiments, thismethod of cross-linking is referred to as thermal cross-linking. Withoutbeing bound by any particular theories, it is believed that, when thewater-absorbing polymer includes a co-polymer of acrylate monomers andacrylamide monomers, the cross-linking can comprise a covalentcross-link that includes the carboxylate anion on the acrylate monomerscovalently bonding to another portion of a water-absorbing polymer, suchas an acrylamide monomer. Further, without being bound by any particulartheories, it is believed that this thermal cross-linking facilitates thesecuring of the polymer coating around the substrate particle. Incertain embodiments, prior to this thermal cross-linking, one or more ofthe water-absorbing polymers in the polymer-coated substrate particlesis linear.

In certain embodiments, the amount of cross-linking of thewater-absorbing polymers can be controlled by the temperature and timeof heat exposure. In such embodiments, the polymer-coated substrateparticles can be exposed to a temperature of at least about 100° F. (38°C.), or at least about 150° F. (66° C.); and/or less than about 250° F.(121° C.), or less than about 230° F. (110° C.); or a temperature ofabout 180° F. (82° F.) for a time period of at least about 10 minutes,at least about 20 minutes, or at least about 30 minutes.

In various embodiments, the amount of cross-linking of thewater-absorbing polymers can be controlled by the anionic content of thewater-absorbing polymers present on the polymer-coated substrateparticles. In such embodiments, one or more of the water-absorbingpolymers can be about 10 mol % to about 50 mol % anionic, or about 20mol % to about 50 mol % anionic. In certain embodiments, thewater-absorbing polymers present in the self-suspending proppantsproduced according to the methods described herein may be partiallycross-linked (e.g., not fully cross-linked). In such embodiments, thispartial cross-linking may provide sufficient water swellability andsuspension of the proppant while still allowing for the ability tosettle out when exposed to a conventional breaker.

In certain embodiments, by controlling the heating temperature, exposuretime to the heat, and anionic content of the water-absorbing polymers onthe outer polymeric coating, self-suspending proppants are formed thatexhibit the desirable suspension properties disclosed herein, includingthe ability to remain suspending in brackish or salt water conditions,while also being able to settle out when exposed to a conventionalbreaker, as discussed further below.

In certain embodiments, two distinct applications of a water-absorbingpolymer in powdered form and in the form of an emulsion can providedifferent coating properties to the self-suspending proppant, which canresult in advantageous performance of the self-suspending proppants. Forexample, applying a water-absorbing polymer in an emulsion or otherliquid can provide a substantially continuous or continuous coating ofthe water-absorbing polymer to the surface of the substrate particle, asthe emulsion or liquid can evenly coat the substrate particle. Further,in certain embodiments, this substantially continuous coating, whensubsequently cross-linked, can form a cross-linked coating on thesurface of the substrate particle, effectively locking the substrateparticle inside this cross-linked polymeric coating. Additionally, incertain embodiments, the application of the water-absorbing polymer indried powdered form can increase the water-absorbing polymer content inthe self-suspending proppant's polymer coating (relative to a singleapplication of the water-absorbing polymer in an emulsion), which mayaid in the suspension ability, including in salt water, while stillallowing for polymer coating removal from the substrate usingconventional breakers.

In certain embodiments, due to the size of the powdered particles of thepowdered form of the water-absorbing polymer relative to the size of thesubstrate, such as various grades of frac sand, and due to the amount ofpowdered polymer added in the methods described herein, individualsubstrates may not be fully coated with powdered polymer particles. Eventhough the substrates may not be fully coated with the powdered polymerparticles in certain embodiments, the self-suspending proppant can alsoinclude a coating of a water-absorbing polymer that was applied in anemulsion, which may substantially or entirely coat the outer surface ofthe substrate particle.

Further, in certain embodiments, utilizing two distinct applications ofthe water-absorbing polymer where each application includes applying thewater-absorbing polymer in a different form (e.g., one application withthe water-absorbing polymer in powdered form and another application inan emulsion), allows for the modulation of the amount of thewater-absorbing polymer present on the substrate particle in aproduction efficient manner. For instance, in such embodiments, anapplication of the water-absorbing polymer in an emulsion form canprovide a high localized concentration of water-absorbing polymer to thesurface of the substrate particles; however, the amount ofwater-absorbing polymer that may be applied to the surface of thesubstrate particles may be limited, as only so much of the emulsion canphysically coat onto the substrate particles without falling off. Thusin such embodiments, an application of the water-absorbing polymer inpowdered form, in addition to the application of a water-absorbingpolymer in an emulsion, can increase the water-absorbing polymer contenton the substrate surface (relative to a single application of thewater-absorbing polymer in emulsion form), which can result inadvantageous self-suspending proppant performance, such as an enhancedsalt water tolerance, and/or an enhanced suspension ability. Further,this increase in the water-absorbing polymer content on the substratesurface (relative to a single application of the water-absorbing polymerin emulsion form) can be achieved in a production efficient manner,e.g., utilizing a single vessel and a single drying and/or crosslinkingstep (e.g., via heat exposure).

Methods for Forming Self-Suspending Proppant Particles: Addition of aFlowing Agent

In various embodiments, once the polymer-coated substrate particles aredried thereby forming the self-suspending proppant particles, a flowingagent may be added to the self-suspending proppant particles to aid inhandling and distribution of the self-suspending proppant particles inhumid environments. In embodiments, the flowing agent can include fumedsilica, sodium aluminosilicate, potassium aluminosilicate, calciumaluminosilicate, synthetic zeolites, natural zeolites, or a combinationthereof. In one embodiment, the flowing agent can include a syntheticsodium aluminosilicate zeolite. In certain embodiments, othercommercially available flowing agents can be used. A non-limiting listof other commercially available flowing agents includes potassiumaluminum silicate, silicon dioxide, calcium silicate, and powderedcellulose.

In embodiments, at least about 0.05 wt. % flowing agent relative to theweight of self-suspending proppant particles can be mixed with or addedto the self-suspending proppant particles, at least about 0.1 wt. %, atleast about 0.25 wt. %, at least about 0.5 wt. %, at least about 0.75wt. %, or at least about 1 wt. %, and/or less than about 5 wt. %, lessthan about 4 wt. %, less than about 3 wt. %, or less than about 2 wt. %.In certain embodiments, about 0.1 wt. % flowing agent relative to theweight of self-suspending proppant particles can be added, about 0.25wt. %, about 0.5 wt. %, about 0.75 wt. %, about 1 wt. %, about 2 wt. %,about 3 wt. %, about 4 wt. %, or about 5 wt. %.

Self-Suspending Proppant Particles

As discussed above, the self-suspending proppant particles describedherein can include a substrate, e.g., sand particle, having an outerpolymeric coating. In embodiments, the outer polymeric coating can bediscontinuous. As used herein a discontinuous outer polymeric coatingrefers to a polymeric coating on an outer surface of a substrate (thatis or is not coated in binder) that covers less than about 80% of thesurface, less than about 70%, less than about 60%, less than about 50%,or less than about 25%. In an alternative embodiment, the outerpolymeric coating can be a continuous outer coating that covers at leastabout 50% of the outer surface of the substrate, at least about 60%, atleast about 70%, at least about 80%, at least about 90%, at least about95%, or at least about 99%.

In embodiments, the self-suspending proppant particles made according tothe methods described above can include more than one application of awater-absorbing polymer to form the outer polymeric coating. In certainembodiments, as discussed above, the self-suspending proppant particlescan include a first water-absorbing polymer that may be applied to asubstrate particle, or binder-coated substrate particle, and a secondwater-absorbing polymer that may be applied to the intermediatepolymer-coated substrate particles.

In certain embodiments, the combined amount of the first water-absorbingpolymer applied to the binder-coated substrate particles and the secondwater-absorbing polymer applied to the intermediate polymer-coatedsubstrate particles can provide an outer polymeric coating weight (orcoat weight) to the self-suspending proppant particles of at least about0.5 wt. % coat weight, at least about 1.0 wt. %, at least about 1.5 wt.%, at least about 2.0 wt. %, at least about 2.5 wt. % or at least about3.0 wt. %, and/or less than about 7.5 wt. %, less than about 5.0 wt. %,or less than about 4 wt. %. As used herein, wt. % coat weight refers tothe ratio of the total weight of the water-absorbing polymer applied tosubstrate, to the weight of the substrate, multiplied by 100.

In certain embodiments, depending on the application of a binder and oneor more water-absorbing polymers, the outer polymeric coating of theself-suspending proppant particles can include glycerol, mineral oil,one or more water-absorbing polymers, or a mixture thereof. In suchembodiments, the water-absorbing polymers can comprise, consistessentially of, or consist of one or more linear polyacrylamides. Incertain embodiments, the one or more linear polyacrylamides can includetwo linear polyacrylamides having different molecular weights and/ordifferent mol. % anionic charges.

In one or more embodiments, depending on the application of a binder andone or more water-absorbing polymers, the outer polymeric coating of theself-suspending proppant particles can include glycerol, mineral oil,one or more water-absorbing polymers, or a mixture thereof. In suchembodiments, the water-absorbing polymers can comprise, consistessentially of, or consist of one or more of polyacrylamides,polyacrylates, or co-polymers of acrylate monomers and acrylamidemonomers.

In various embodiments, the first and second water absorbing polymerscan be different from one another. For example, in one embodiment, thefirst and second water absorbing polymers can be differentpolyacrylamide polymers of different molecular weights. In such anembodiment, the second water-absorbing polymer, e.g., polyacrylamidepolymer, may have a higher molecular weight than that of the firstwater-absorbing polymer, e.g., a polyacrylamide polymer.

In certain embodiments, the first and second water-absorbing polymerscan be different types of polymers, or similar types of polymers withdifferent properties. For example, the first and second water-absorbingpolymers can be the same types of polymers, e.g., polyacrylamides, butone may be a linear, non-cross-linked polymer, and the other polymer maybe cross-linked. In another example, the first and secondwater-absorbing polymers can be different types of polymers, such as onebeing a polyacrylamide and the other being a polyacrylate or co-polymerof acrylate and acrylamide.

In embodiments, the ability to vary the types or properties of thewater-absorbing polymers applied to the substrate can impart variousbeneficial properties to the self-suspending proppant particles. Forexample, in certain embodiments, by using a cross-linked water-absorbingpolymer for at least one of the water-absorbing polymers in the outerpolymeric coating one can increase the ability for such aself-suspending proppant particle to remain suspending in brackish waterconditions that may be found in various wells.

In one embodiment, a self-suspending proppant having a firstwater-absorbing polymer comprising linear polyacrylamide (which may beapplied via a water-in-oil emulsion) and a second water-absorbingpolymer comprising a crosslinked polymer, such as crosslinkedpolyacrylate or a crosslinked co-polymer of acrylamide and acrylate(which may be applied by to the substrate in powdered form) can greatlyincrease the suspension of the proppant in various types of salt orbrackish water.

In another embodiment, a self-suspending proppant having a firstwater-absorbing polymer comprising a first linear polyacrylamide and asecond water-absorbing polymer comprising a second linear polyacrylamidethat is different from the first linear polyacrylamide, or is applied tothe substrate differently than the first linear polyacrylamide, cangreatly increase the suspension of the proppant in various types of saltor brackish water.

In one or more embodiments, the self-suspending proppants made asdescribed herein may remain suspended in a 1000 ppm CaCO₃ aqueoussolution for at least 30 minutes, at least 60 minutes, at least 90minutes, or at least 120 minutes, at room temperature or, alternativelyat a temperature of 170° F., after adding the self-suspending proppantparticles to the salt solution and shaking to incorporate.

In certain embodiments, a breaker, such as ammonium persulfate or sodiumchlorite, may be present at varying levels in a suspension test, such asthat described above. The addition of a breaker may be utilized todetermine if the proppant will become suspended, or if alreadysuspended, to determine if the suspended proppant can be “broken” (andsettle out). In embodiments, the breaker may be present in an amount offrom 1-100 pounds per thousand gallons.

In certain embodiments, by varying the types and/or properties of thefirst and second water-absorbing polymers utilized in forming theself-suspending proppant particles one may be able to tailor aparticular type of self-suspending proppant particle for a particulartype of water, such as water containing cations, chlorine, iron, orother ionic components.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrative,and not restrictive. The scope of the invention is, therefore, indicatedby the appended claims, rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

The concepts discussed herein will be further described in the followingexamples, which do not limit the scope of various embodiments describedin the claims.

EXAMPLES Example 1: Effect of Various Brackish Well Waters on SpecificSelf-Suspending Proppants

In this Example, various self-suspending proppants were made andanalyzed for their ability to remain suspended in various brackish watersamples from fracking wells. Each of the proppants tested in this methodhave been made according to the methods described above. For example,frac sand (40-70 mesh frac sand, unless otherwise noted below) was addedto a mixing device, e.g., a kitchen mixer, and 0.20 wt. % glycerol wasadded and allowed to mix for two to three minutes. Then a water-in-oilemulsion containing a linear polyacrylamide was added and allowed to mixfor two to three minutes, followed by the addition of another linearpolyacrylamide in powder form, which was mixed in for a period of two tothree minutes. The amounts of water-in-oil emulsion and powdered linearpolyacrylamide that were added in the various samples are provided belowin Table 1. The proppants were then dried at about 230° F. for 15-20minutes in an electric frying pan.

TABLE 1 Water-absorbing polymers components in self-suspending proppantsamples Amount of Water- Powdered absorbing polyacrylamide polymerAmount of (Hyperfloc ® AF coat weight emulsion 308 product) Sample (wt.%)* Emulsion (wt. %) (wt. %) A 1.5 Hyperfloc ® 3.33 0.5 AE 873H B 2.0Hyperfloc ® 3.33 1.0 AE 853H C 2.5 Hyperfloc ® 3.33 1.5 (30-50 mesh AE853H frac sand) D 2.0 Hyperfloc ® 3.33 1.16 AE 852 E 1.5 Hyperfloc ® 2.00.5 AE 859 F 1.5 Hyperfloc ® 3.33 0.66 AE 872 *wt. % of coat weightrefers to the ratio of the weight of the water absorbing polymer added(from both the emulsion and the powdered polyacrylamide) to the weightof the frac sand, multiplied by 100. The weight of water absorbingpolymer from the emulsion was determined based on the weight percent ofpolyacrylamide in the emulsion. The emulsions and powderedpolyacrylamide were obtained from SNF.

11.88 grams of each of the self-suspending proppants was added to 45 mLof various test waters in a 50 mL tube (2.2 ppg), capped, and thenshaken by hand vigorously for thirty seconds. The shaken tubes wereobserved over time the settling of the self-suspending proppants. Theresults appear below in Tables 2-4.

TABLE 2 Suspension results in brackish well water sample #1 Well watersample # 1: 11.73 mS conductivity and pH of 8.34 First Second ThirdFourth Fifth reading: reading: reading: reading: reading: Time and Timeand Time and Time and Time and Suspension Suspension SuspensionSuspension Suspension Sample Level* Level* Level* Level* Level* A 0min.-100% 5 min.-37 mL 18 min.-42 mL 60 min.-43 mL 120 min.-44 mL (6 mL)B 0 min.-100% 5 min.-22 mL 10 min.-25 mL 47 min.-27 mL 107 min.-28 mL (6mL) C 0 min.-100% 58 min.-28 mL  n/a n/a n/a (6 mL) D 0 min.-100% 5min.-20 mL 10 min.-25 mL 42 min.-28 mL  102 min.-33.5 mL (6 mL) E 0min.-100% 5 min.-31 mL 37 min.-34 mL 97 min.-35 mL n/a (6 mL) F 0min.-100% 32 min.-45 mL  92 min.-46 mL n/a n/a (6 mL) *Suspension levelwas visually determined based on the position of the top of thesuspended proppants in the brackish well water sample #1. For example,100% suspended refers to seeing no settled proppant and the position ofthe top of the suspended proppants is approximately at the 6 mL level,as determined based on the graduations on the tube. 28 mL is determinedto be 50% suspended.

TABLE 3 Suspension results in brackish well water sample #2 Well watersample # 2: 1024 μS conductivity and pH of 7.83 First Second ThirdFourth reading: reading: reading: reading: Time and Time and Time andTime and Suspension Suspension Suspension Suspension Sample Level*Level* Level* Level* A 0 min.-100% 44 min.-10 mL 18 min.-42 mL 60min.-43 mL (6 mL) B 0 min.-100% 5 min.-6 mL 49 min.-7 mL  n/a (6 mL) C 0min.-100% 7 min.-1 mL 63 min.-15 mL n/a (6 mL) D 0 min.-100% 5 min.-6 mL53 min.-7 mL  n/a (6 mL) E 0 min.-100% 5 min.-8 mL 12 min.-11 mL 60min.-28 mL (6 mL) F 0 min.-100%  4 min.-30 mL 11 min.-33 mL 64 min.-41mL (6 mL) *Suspension level was visually determined based on theposition of the top of the suspended proppants in the brackish wellwater sample #2. For example, 100% suspended refers to seeing no settledproppant and the position of the top of the suspended proppants isapproximately at the 6 mL level, as determined based on the graduationson the tube. 28 mL is determined to be 50% suspended.

TABLE 4 Suspension results in brackish well water sample #3 Well watersample # 2: 3.62 mS conductivity and pH of 8.27 First Second ThirdFourth reading: reading: reading: reading: Time and Time and Time andTime and Suspension Suspension Suspension Suspension Sample Level*Level* Level* Level* C 0 min.-100% 3 min.-21 mL 24 min.-21 mL 92 min.-26mL (6 mL) *Suspension level was visually determined based on theposition of the top of the suspended proppants in the brackish wellwater sample #3. For example, 100% suspended refers to seeing no settledproppant and the position of the top of the suspended proppants isapproximately at the 6 mL level, as determined based on the graduationson the tube. 28 mL is determined to be 50% suspended.

TABLE 5 Suspension results in fresh water sample Fresh Water Sample:2.43 mS conductivity and pH of 8.05 First Second Third reading: reading:reading: Time and Time and Time and Suspension Suspension SuspensionSample Level* Level* Level* C 0 min.-100% 21 min.-16 mL 90 min.-24 mL (6mL) *Suspension level was visually determined based on the position ofthe top of the suspended proppants in the fresh water sample. Forexample, 100% suspended refers to seeing no settled proppant and theposition of the top of the suspended proppants is approximately at the 6mL level, as determined based on the graduations on the tube. 28 mL isdetermined to be 50% suspended.

As can be seen in the above results, Samples B and C, which included thesame water-absorbing polymers, just with varying coat weight,unexpectedly appeared suspended longer than the other samples (asidefrom Sample D). For example in brackish water samples #1 and #2, atleast 50% of Samples B and C were suspended after about one hoursubsequent to shaking. Further, Sample C performed equally as well inbrackish water sample #3 and the fresh water sample. In any of the fourwater samples tested, Sample C was not observed to be less than 50%suspended within the observed timeframe. Although Sample D stayed almost100% suspended in brackish water sample #2 about 60 minutes aftershaking, the same sample was observed to be less than 50% suspended inbrackish water sample #1 after about 102 minutes.

Example 2: Self-Suspending Proppants in 1000 ppm CaCO₃

In this Example, the suspension of various self-suspending proppantswere analyzed in 1000 ppm CaCO₃. In this Example two main samples wereutilized, a self-suspending proppant having a cross-linkedwater-absorbing polymer and another self-suspending proppant with onlylinear water-absorbing polymers. The self-suspending proppant havingonly linear water-absorbing polymers was made as described in Example 1with reference to Sample B.

For the self-suspending proppant having a cross-linked water-absorbingpolymer, frac sand (40-70 mesh) was added to a mixing device, a kitchenmixer, and 0.26 wt. % glycerol was added and allowed to mix for lessthan two minutes. Then 0.45 wt. % SNF Hyperfloc® AE 873H linearpolyacrylamide emulsion was added and allowed to mix for two to threeminutes, followed by the addition of 2 wt. % of an acrylamide andpotassium acrylate co-polymer, cross-linked (Evonik IndustriesStockosorb® S 18G) in powder form, which was mixed in over a period ofabout five minutes. Next an additional 0.45 wt. % % SNF Hyperfloc® AE873H linear polyacrylamide emulsion was added and allowed to mix for twoto three minutes. The proppants were then dried at 200° F. for 30minutes in a conventional oven and 0.5 wt. % fumed silica was added.

Approximately the same amount of proppants were added to separate vialscontaining a 1000 ppm CaCO₃ solution and shaken vigorously for thirtyseconds and allowed to sit in an oven at a temperature of 170° F. for 75minutes. An additional vial contained the linear polyacrylamideself-suspending proppant and the same 1000 ppm CaCO₃ solution along witha breaker and was treated similarly. After 75 minutes at 170° F., bothself-suspending proppants remained suspended in the brackish conditions.In addition, the vial containing the linear polyacrylamideself-suspending proppant and the breaker settled out, suggesting thatsuch an outer polymeric coating can be “broken” with a breaker to settleout the proppant. The cross-linked self-suspending proppant was notexposed to the breaker.

Example 3: Effects of Various Flowing Agents on Self-SuspendingProppants in Humid Conditions

In this Example, various flowing agents were tested with aself-suspending proppant in humid conditions to test for flowability andwater absorption. Table 6 below lists the various self-suspendingproppant samples and the various flow treatments.

TABLE 6 Flowability Test Proppant Samples Base Flow Mix Sand CoatingFlow Agent sand Sample (mesh) Mixture Drying Profile Agent (wt. %) (wt.%) A 30/50 A-1.5% 220° F.-15 min None — 40/ 70-10% B 30/50 A-1.5% 220°F.-15 min E-554 0.50% — C 40/70 A-1.5% 220° F.-15 min DFC 0.17% — D40/70 A-1.5% None E-554 0.50% — E 40/70 A-1.5% 220° F.-15 min Whey 1.00%— F 30/50 A-1.5% 220° F.-15 min WPC 34 1.00% — G 30/50 A-1.5% None Whey1.00% — H 40/70 A-1.5% None WPC 34 1.00% — I 40/70 A-1.5% None E-5541.00% — J 40/70 A-1.5% None E-554 2.00% — K 30/50 A-1.5% 220° F.-15 minNone — — L 30/50 A-1.5% 220° F.-15 min E-554 1.00% — M 40/70 A-1.5% 220°F.-15 min DFC 0.12% — N 40/70 A-1.5% 220° F.-15 min Whey 1.50% — O 40/70A-1.5% 220° F.-15 min WPC 34 1.50% — P 40/70 A-1.5% 220° F.-15 min F0.10% — Q 40/70 A-1.5% 220° F.-15 min F 0.15% — R 40/70 A-1.5% None None— —

The proppant samples listed in Table 6 were prepared as described abovewith respect to sample A of Table 1 in Example 1 to give a coated sandwith a 1.5% by weight outer polymeric coating. The same polyacrylamidesutilized in sample A of Table 1 in Example 1 were utilized. However, thedrying protocol differed as described in Table 6, for example some ofthe samples were dried with a standard temperature profile, while somewere not heat treated. Some of the samples were treated with a flowagent product, as listed in Table 6. Some of the samples were mixed withuncoated sand. The “E-554 (Zeolex 23A)” flow agent is a sodiumaluminosilicate purchased from J.M. Huber Corporation. The “F” flowagent is Aerosil R202 fumed silica purchased from Evonik Corporation.The “WPC 34” is a whey protein concentrate at 34% that is commerciallyavailable. The “whey” is commercially available. DFC is a proprietarycoating utilized to provide a dust-free frac sand.

First, the various proppant samples were exposed to a humidity chamber.Specifically, the samples prepared as in Table 6 were added to an openvial and weighed. Then, the vials were exposed to 85% relative humidityat 90° F. for 15 minutes and weighed thereafter. The temperature andhumidity were verified by sensor recordings. The results are listed inTable 7 below.

TABLE 7 Humidity Testing in vials Weight Trial when Pre-Trial Post-TrialWeight prepared Weight Weight Gain Sample (g) (g) (g) (g) A 41.23 41.2341.24 0.01 B 42.94 42.93 42.95 0.02 C 39.52 39.52 39.53 0.01 D 35.8235.81 35.81 0 E 41.95 41.94 41.95 0.01 F 42.3 42.29 42.31 0.02 G 31.9131.92 31.92 0 H 30.56 30.57 30.56 −0.01 I 36.74 36.74 36.74 0 J 39.6339.62 39.63 0.01 K 40.65 40.65 40.65 0 L 41.05 41.04 41.06 0.02 M 43.1343.11 43.11 0 N 41.61 41.6 41.62 0.02 O 39.02 39.02 39.04 0.02 P 38.0138.02 38.02 0 Q 37.88 37.88 37.9 0.02 R 29.95 29.9 29.9 0

Next, the samples exposed to the humidity chamber as above were testedfor their ability to flow out of a vial. Specifically, each sample vialwas emptied into the Petri dish. The number of shakes/taps of theupside-down vial required to empty the sand from the vial to the Petridish were recorded. Each time further agitation is required to break upvial sand clumps add 25 shakes/taps to the count. If the equivalent of100 or more shakes are required, record 100 shakes/taps and empty theremaining contents into the Petri Dish by any means. 4. If a surfaceclump was formed on the top exposed surface of the vial, record therelative size of the surface clump. The level of bulk clumping visiblein each sample, and whether the sand of the sample is uniformly stickingtogether was recorded. The results appear in Table 8 below and FIG. 9shows the images of the petri dishes.

TABLE 8 Flow testing from vials Shake/Tap Surface Sample Scale ClumpBulk Sticky A 5 Medium B 5 Medium C 10 Medium Small D 1 Small E 5 MediumF 2 G 100 Bulk Large Yes H 75 Bulk Large Yes I 0 J 0 K 3 Small Small L 1M 0 Small N 2 Large O 8 Large Small P 1 Small Q 1 Small R 100 Bulk LargeYes

Next the petri dishes from FIG. 9 containing the samples A-R, weresubjected to the humidity chamber again, this time for 60 minutes at 90°F. with 85% relative humidity. The temperature and humidity wereverified by sensor recordings. The petri dishes were weighed before andafter this humidity chamber exposure. The results are listed in Table 9below.

TABLE 9 Humidity testing in petri dish Trial Pre-Trial Post-Trial WeightWeight Weight Gain Sample (g) (g) (g) A 40.62 40.88 0.26 B 42.29 42.550.26 C 38.87 39.1 0.23 D 35.12 35.17 0.05 E 41.36 41.65 0.29 F 41.6441.93 0.29 G 31.18 31.22 0.04 H 29.91 29.92 0.01 I 36.04 36.1 0.06 J38.93 38.99 0.06 K 39.97 40.16 0.19 L 40.23 40.51 0.28 M 42.22 42.490.27 N 40.84 41.13 0.29 O 38.4 38.66 0.26 P 37.28 37.52 0.24 Q 37.0737.31 0.24 R 28.55 28.52 −0.03

The samples in the petri dish were dumped over onto white pieces ofpaper and are shown in FIG. 10.

Next the samples on the white pieces of paper were evaluated forflowability by being transferred back to their original vials using aplastic funnel. Specifically, an empty funnel was placed into thesample's original sample vial and the sand sample was slid off the paperinto the sample funnel. The funnel was shaken vertically 15 times tobreak up the sand sample in an attempt to have the sand gravity flowinto the sample vial. If the humidified sand sample formed a Petridished shape clump when slid into the funnel, it was noted how easilythe clump breaks up when during the shaking. Further, it was noted howeasily the sample sand flowed into the vial from the funnel once theinitial clump was broken up (if present). The approximate fill volume ofsample sand that was returned to each of the original sample vials wasrecorded. The overall flow and transfer of humid sand was scoredaccording to the following Table 10 and the results are listed in Table11.

TABLE 10 Scoring for the flow transfer return to vials Points ScoredClump Breakage Funnel Flow Vial Fill 20 No Clump 15 Very Easy Very Easy90-110% 10 Easy Easy  75-90% 5 Slightly Difficult Slightly Difficult 50-75% 2 Difficult Difficult 25%-50%  1 Very Difficult Very Difficult  <25% 0 No Breakage No Flow    0%

TABLE 11 Flow transfer return from vials Sample Clump Breakage FunnelLoading Flow Vial Fill Score I No Clump Very easy 90-110%   50 J NoClump Very easy 90-110%   50 D No Clump Easy 90-110%   45 B Very EasyEasy 90-110%   40 L Very Easy Easy 90-110%   40 P Very Easy Easy90-110%   40 Q Very Easy Easy 90-110%   40 K Easy Difficult 50-75%  17 OSlightly Difficult Difficult 50-75%  12 A Difficult Difficult <25% 5 EDifficult Difficult <25% 5 F Difficult Difficult <25% 5 M DifficultDifficult <25% 5 N Difficult Difficult <25% 5 C Very Difficult VeryDifficult <25% 3 G Very Difficult Very Difficult <25% 3 H Very DifficultVery Difficult <25% 3 R Very Difficult Very Difficult <25% 3

Based on the above results from Tables 7-11 a humidity and flowcorrelation analysis was performed, which is shown in Table 12.

TABLE 12 Mixture variation performance analysis Drying Profile 220°F.-15 min None Flow Weight Tap Flow Weight Tap Score Gain Scale ScoreGain Scale Flow Agents AVG AVG AVG AVG AVG AVG E-554 40.0 0.79% 3 48.3  0.19% 0 sample B 40.0 0.74% 5 sample D 45.0   0.18% 1 sample I 50.0  0.21% 0 sample J 50.0   0.19% 0 sample L 40.0 0.84% 1 F 40.0 0.80% 1sample P 40.0 0.79% 1 sample Q 40.0 0.80% 1 None 11.0 0.67% 4 3.0 −0.14%100 sample A 5.0 0.77% 5 sample K 17.0 0.58% 3 sample R 3.0 −0.14% 100WPC 34 8.5 0.83% 5 3.0   0.04% 75 sample F 5.0 0.84% 2 sample H 3.0  0.04% 75 sample O 12.0 0.83% 8 Whey 5.0 0.85% 4 3.0   0.17% 100 sampleE 5.0 0.84% 5 sample G 3.0   0.17% 100 sample N 5.0 0.86% 2 DFC 4.00.74% 5 sample C 3.0 0.72% 10 sample M 5.0 0.77% 0 Grand Total 18.10.78% 4 25.7   0.11% 46

Next various samples were subjected to centrifugal settling. Theproppant samples are listed below in Table 13. The samples were made asdiscussed above with respect to the samples in Table 6.

TABLE 13 Proppant samples for centrifugal compression Flow Agent Added(% by weight) Sample E-554-sodium ID Base Preparation aluminosilicate W40/70, A-1.5% coat weight, 0.5% Undried X 40/70, A-1.5% coat weight,1.0% Undried V 40/70, A-1.5% coat weight, 2.0% Undried

For each of the above samples, a 2.1 g standard method sample wasprepared of 40/70 mesh dried sand coated with the standard polymermixture, dried using the standard temperature profile, and 0.1% weightpercent of Product F (fumed silica) added.

2.1 g of each of samples W, X, and V (and the standard samples) washydrated, by placing it in 8 ml of water inside its own 15 ml centrifugetube. Each tube was then shaken vigorously to mix the proppant and watermixture and then centrifuged at a low speed for 2-5 minutes. A speed a13% was used for this testing. The centrifuge was an IEC InternationalCentrifuge Model HT. This centrifuge did not have workinginstrumentation to show its rotational speed. According to associatedliterature, this model centrifuge should have a maximum speed of 17,000RPM and generate a relative centrifugal force of 34375. After spinningat 20% speed the rotor RPM was tested with a separate instrument whichgave a value of 3300 RPM. This is consistent with a maximum 100% speedof 17,000 RPM. After centrifuging, the samples were removed and thesettling of the materials was compared.

No difference in settling was noted in the samples spun for 5 minutes.The Sample X spun for 3 minutes showed slightly less settling than thestandard method sample. The undried samples with the sodiumaluminosilicate had more opaque gel layer than that of the standardsamples.

Finally, another centrifugal compression test was performed. The samplesfor this test were prepared according to the below Table 14.

TABLE 14 Centrifuge compression samples Base Flow Sand Coating DryingFlow Agent Mix Taps to Sample (mesh) Mixture Profile Agent Amount SandRemove B 30/50 A-1.5% 220° F.- E-554 0.50% 1 15 min I 40/70 A-1.5% NoneE-554 1.00% 9 K 30/50 A-1.5% 220° F.- None 40/ 0 15 min 70-20% Q 40/70A-1.5% 220° F.- F 0.15% 3 15 min

The samples were prepared as indicated in the Table 14 and according tothe procedures set out with respect to Table 6. In this test, 5 ml ofeach sample proppant was placed into a 15 ml centrifuge tube andcentrifuged at a medium or high speed for about 5 minutes (forcentrifuge force see the above description with respect to Table 13. Thesamples were removed and the compaction of the materials was compared.The samples were removed by slowly inverting the centrifuge tube above awhite paper. If some compressed sand was stuck in the centrifuge tube,the tube was lightly tapped on the table until the sand was removed. Anyrequired taps in recorded in the above Table 14. FIGS. 11A and 11B showthe results of the initial dump (FIG. 11A) and the sand fully removedafter tapping (FIG. 11B).

In view of these results of Example 3, the flowing agent that worked thebest to promote flowability of the coated sand when exposed to highhumidity environments is the sodium aluminosilicate (E-554). Unlike thefumed silica samples, the sodium aluminosilicate-containing samplesshowed unexpectedly beneficial properties, such as having coated sandproduct flowability even when added to product that was not heat treatedwith a drying process.

The amount of water vapor absorbed by the coated sand mixtures was notchanged significantly by the flowing agents added. If the coated sandwas not heat treated by drying after coating, it didn't absorb as muchmoisture in the humid environment. When exposed to an environment of 90°F. and 85% relative humidity for 60 minutes the undried samplesincreased weight an average of 0.1% compared to an average weightincrease of 0.8% for the dried samples.

What is claimed is:
 1. A method for making self-suspending proppantparticles, the method comprising: mixing substrate particles with aliquid binder so that at least a portion of the substrate particles areat least partly coated with the liquid binder; applying one or morewater-absorbing polymers to the at least a portion of the substrateparticles that are at least partly coated with liquid binder to formpolymer-coated substrate particles, wherein the applying one or morewater-absorbing polymers includes applying a first water-absorbingpolymer in a first form, and applying a second water-absorbing polymerin a second form that is different from the first form, wherein thefirst and second forms are selected from a powder and an emulsion, andwherein each of the first and second water-absorbing polymers is about20 mol. % anionic to about 50 mo. % anionic; and exposing thepolymer-coated substrate particles to heat sufficient to causecrosslinking in at least a portion of the one or more water-absorbingpolymers present on the polymer coated substrate particles, therebyforming self-suspending proppant particles.
 2. The method according toclaim 1, wherein the first water-absorbing polymer comprises a firstco-polymer of acrylate monomers and acrylamide monomers, wherein thefirst form of the first water-absorbing polymer is the powder form. 3.The method according to claim 2, wherein the second water-absorbingpolymer comprises a second co-polymer of acrylate monomers andacrylamide monomers, wherein the second form of the secondwater-absorbing polymer is the emulsion, wherein the emulsion is awater-in-oil emulsion, wherein, in the water-in-oil emulsion, the secondwater-absorbing polymer is in an aqueous phase that is dispersed in anoil phase.
 4. The method according to claim 1, a molecular weight of thefirst water-absorbing polymer is less than a molecular weight of thesecond water-absorbing polymer.
 5. The method according to claim 4,wherein the molecular weight of the first water-absorbing polymer is atleast about 1 Million Daltons (g/mol) less than the molecular weight ofthe second water-absorbing polymer.
 6. The method according to claim 1,wherein the first and second water-absorbing polymers are linear priorto the applying heat to the polymer-coated substrate particles.
 7. Themethod according to claim 1, wherein each of the first and secondwater-absorbing polymers is at least about 30 mol. % anionic to about 40mol. % anionic.
 8. The method according to claim 2, wherein the powdercomprises powder particles with a maximum dimension of from about 50microns to about about 300 microns.
 9. The method according to claim 1,wherein the liquid binder comprises glycerol.
 10. The method accordingto claim 9, wherein the glycerol is present in the self-suspendingproppant particles.
 11. The method according to claim 3, wherein theapplying one or more water-absorbing polymers to the at least a portionof the substrate particles comprises applying the first water-absorbingpolymer in the first form prior to applying the second water-absorbingpolymer in the second form.
 12. The method according to claim 3, whereinthe applying one or more water-absorbing polymers to the at least aportion of the substrate particles comprises applying the secondwater-absorbing polymer in the second form prior to applying the firstwater-absorbing polymer in the first form.
 13. The method according toclaim 1, wherein the exposing the polymer-coated substrate particles toheat comprises exposing the polymer-coated substrate particles to atemperature of less than about 200° F. (93° C.).
 14. The methodaccording to claim 1, further comprising adding a flowing agent to theself-suspending proppant particles.
 15. The method according to claim14, wherein the flowing agent comprises a sodium aluminosilicate. 16.The method according to claim 1, wherein the self-suspending proppantparticles are configured to remain suspended in a 1000 ppm CaCO₃ aqueoussolution for at least 30 minutes at a temperature of 170° F.
 17. Amethod for making self-suspending proppant particles, the methodcomprising: adding a volume of substrate particles to a mixing vessel,the substrate particles comprising sand; coating at least a portion ofthe substrate particles with glycerol in the mixing vessel to formbinder-coated substrate particles, mixing a first water-absorbingpolymer with the binder-coated substrate particles in the mixing vessel,the first water-absorbing polymer comprising a co-polymer of acrylamidemonomers and acrylate monomers, wherein the first water-absorbingpolymer is about 20 mol. % anionic to about 50 mo. % anionic, andwherein the first water-absorbing polymer is in powdered form;subsequent to mixing the first water-absorbing polymer with thebinder-coated substrate particles, mixing a second water-absorbingpolymer with first water-absorbing polymer and the binder-coatedsubstrate particles to form polymer-coated substrate particles, thesecond water-absorbing polymer comprising a co-polymer of acrylamidemonomers and acrylate monomers, wherein the second water-absorbingpolymer is about 20 mol. % anionic to about 50 mo. % anionic, whereinthe second water-absorbing polymer is present in a water-in-oilemulsion, and wherein each of the polymer-coated substrate particlescomprises a polymeric coating that includes the first water-absorbingpolymer and the second water-absorbing polymer; and exposing thepolymer-coated substrate particles to heat sufficient to causecrosslinking in the polymeric coating present on the polymer-coatedsubstrate particles, thereby forming self-suspending proppant particles.18. The method according to claim 17, wherein each of the first andsecond water-absorbing polymers are linear polymers prior to theexposing the polymer-coated substrate particles to heat.
 19. The methodaccording to claim 17, wherein the self-suspending proppant particlesare configured to remain suspended in a 1000 ppm CaCO₃ aqueous solutionfor at least 30 minutes at a temperature of 170° F.
 20. Self-suspendingproppant particles, comprising: substrate particles, the substrateparticles comprising sand; and an outer polymeric coating positioned onan outer surface of each of the substrate particles, the outer polymericcoating comprising first and second water-absorbing polymers that are atleast partly covalently crosslinked, wherein each of the first andsecond water-absorbing polymers comprise a co-polymer of acrylatemonomers and acrylamide monomers, wherein a molecular weight of thefirst water-absorbing polymer is at least about 1 Million Daltons(g/mol) greater than a molecular weight of the second water-absorbingpolymer, wherein each of the first and second water-absorbing polymersis about 20 mol. % to about 50 mol. % anionic, wherein the firstwater-absorbing polymer was applied to the substrate particles in theform of an emulsion, and wherein the second water-absorbing polymer wasapplied to the substrate particles in powdered form, wherein theself-suspending proppant particles remain suspended in a 1000 ppm CaCO₃aqueous solution for at least 30 minutes at a temperature of 170° F.